1. Field of the Invention
The present invention relates to a ceramic heater obtained by hot-pressing a ceramic powder containing a heater main body embedded therein to sinter the powder, and a producing method thereof.
2. Description of the Related Art
Ceramic heaters have been known which are obtained by fixing one-side ends of two lead-out tungsten wires respectively to both ends of a U-shaped metallic heating material (made of a tungsten alloy), embedding the resultant heater main body in a ceramic powder comprising Si.sub.3 N.sub.4, Sialon, or AlN as the main component, and hot-pressing the powder containing the heater main body to sinter the powder.
These ceramic heaters are used in ceramic glow plugs to be fitted to diesel engines. In producing such a ceramic glow plug, a cylindrical main metallic shell is used which has at the front end thereof a holding part extending inward and in a rear part thereof a screw thread for fitting to an engine. A ceramic heater of the above-described kind is fitted into the holding part of the main metallic shell through a metallic sheath.
However, in the prior art process for producing a ceramic heater (during hot-pressing for sintering), carbon of a carbon mold and/or carbon contained in an organic binder comes into the materials being hot-pressed to thereby form a layer of a carbon/tungsten reaction product on the surfaces of the lead-out tungsten wires. As a result, for example, the heater main body (composed of lead-out wires and a metallic heating material) suffers an durability endurance reduction and a resistance increase, and the ceramic develops cracks.